Optical fiber cables containing a plurality of optical fibers for the transmission of optical signals are well known. Such optical fiber cables typically include a core which may have a strength member to carry the axial tensile stress and axial compressive forces on the cable. Also located within the core are one or more tubes. Each tube typically includes a plurality of optical fibers. The optical fibers within a tube may be individually stranded or may be provided in an optical fiber ribbon. A sheath is provided to enclose the core including the tubes and the strength member. The optical fibers included within such a cable typically include a glass core and one or more claddings and/or coatings.
During a process of manufacturing a glass optical fiber, a glass fiber is drawn from a preform and then coated with one or more coating materials, typically ultra-violet light curable materials. The coating materials include, for example, polymeric compositions and are applied by one or more coating applicators. The function of the fiber coating is to protect the surface of the glass optical fiber from mechanical scratches and abrasions which the optical fiber may experience during subsequent handling and use. The coating or coatings also influence the fiber's optical characteristics in response to external mechanical forces and environmental temperature.
Optical fibers are almost universally color-coded in their end use. There are numerous colors which are acceptable in most markets, with additional identification being made possible by "banding" colored fibers with additional colors or circumferential striping. One well-known method of coloring an optical fiber is to apply an ink layer to an optical fiber having single or dual coating layers so that the total composite optical fiber includes primary and secondary coating layers with an outermost ink layer. The ink coloring layer is thin, typically 3 to 5 microns in thickness, and typically includes a carrier resin and a pigment system. The carrier resin may typically be a soluble thermoplastic material or a ultra-violet (UV) curable resin. In the former, the ink is applied via a dye or a transfer method, such as a felt-tip applicator or roller, and the solvent for the carrier resin is driven off by heat to leave the pigmented resin on the fiber. In the UV system, there is no solvent. The liquid resin pigment is cured to a solid state by UV energy. Either ink involves a separate step from either optical fiber production or the cabling operation.
An alternative method for color-coding the fiber is to have the color mixed directly into a secondary (outer) coating of a dual coated optical fiber. The secondary coating acts as the carrier resin for the coloring agents.
One desirable color used for optical fiber coloration is black, and also slate-colored derivations thereof. It is well known in the art to use carbon based black pigment blends in a coloring layer over a fiber having single or dual coating layers to obtain a black or slate color for identification of the optical fiber within a telecommunications cable or ribbon. However, there are several problems associated with using such a carbon based black pigment coloring layer. First, the carbon material absorbs light in the UV region. This presents a potentially significant problem if the primary and secondary coating layers are made of a UV curable material which is not completely cured prior to the application of the color layer. The absorption of UV light by the color layer inhibits a complete cure of the coatings on the optical fiber during drawing of the optical fiber.
A second problem associated with the use of a carbon based black color layer is that the absorption of light by the carbon in the color layer inhibits the use of optical fiber fusion splicing equipment. One well-known method of fusing two lengths of optical fiber is to use a fuse and splice apparatus which automatically aligns and splices two lengths of optical fiber. With the automatic alignment, two lengths of optical fiber to be spliced are bent, for example about a mandril, on either side of the intended splice location. Light is injected into one of the fibers at the location of the bend. The injected light passes through the splice location and is detected at the bend location of the second fiber. The device aligns the fibers for fusion at the splice by determining the alignment of the optical fibers for maximum light transmission. The problem associated with using the carbon black coloring for optical fibers is that the carbon absorbs the injection light, which is typically injected at a wavelength of approximately 1300 nm. This light absorption by the carbon black coloring results in a very weak signal, or no signal, being passed through the fiber for purposes of alignment, thereby aggravating the problems associated with aligning and fusion splicing fibers.
The carbon black coloring materials currently used in the industry are selected to meet the tolerances specified for color distinguishability in industry standards, such as the standards established by the Electronic Industries Association, EIA/TIA-359-A entitled EIA Standard Colors for Color Identification and Coding, January, 1985 and in EIA/TIA-598 entitled Color Coding of Fiber Optic Cables, April 1992. It would be desirable to provide a black appearing coloration for an optical fiber which meets industry requirements for color distinguishability and which does not absorb UV light, thereby allowing the black colored appearing optical fiber to properly cure and to operate with a fusion splicing device which utilizes automatic alignment of fibers by launching UV light into the optical fibers.